Radiating coaxial cable and method of manufacture thereof

ABSTRACT

The crests of the corrugated outer conductor of a coaxial cable are partially removed along the length of one portion of its circumference to produce apertures in the corrugation crests while leaving the corrugation roots intact. The cable is thus made &#39;&#39;&#39;&#39;leaky&#39;&#39;&#39;&#39; for use as a radiator for tunnel communications systems and the like. Desirable aperture sizes and shapes are described.

United States Patent oltum, Jr.

[451 Sept. 12,1972

1541 RADIATING COAXIAL CABLE AND METHOD OF MANUFACTURE THEREOF [72]Inventor: Alfred G. l'loltum, Jr., Oak Forest,

[73] Assignee: Andrew Corporation,

Park, Ill.

22 Filed: Sept. 14,1970

21 App1.No.: 71,804

Orland [52] US. Cl ..333/84 R, 343/771, 29/600,

333/95 A, 333/96 [51] Int. Cl......H0lp l/00, l-I0lp 11/00, HOlq 13/22[58] Field of Search.....343/770, 771, 905; 333/95 A,

[56] References Cited UNITED STATES PATENTS 2,576,835 11/1951 Hewitt,Jr. ..333/95 A 2,633,532 3/1953 Sichak ..333/96 X Harvey et al...343/770 2,838,735 6/1958 Davis ..333/31 3,077,569 2/1963 Ikrath..333/95 S 3,221,331 11/1965 Spitz ..333/95 S 3,287,490 1l/1966 Wright..333/96 X OTHER PUBLICATIONS De Keyser et al., Radiocommunication &Control in Mines & Tunnels, Electronics Letters 1l-26-70, pp. 767- 768Primary Examiner-Herrnan Karl Saalbach Assistant Examiner-Wm. H. PunterAttorney-Leonard G. Nierman [5 7] ABSTRACT The crests of the corrugatedouter conductor of a coaxial cable are partially removed along thelength of one portion of its circumference to produce apertures in thecorrugation crests while leaving the corrugation roots intact. The cableis thus made leaky for use as a radiator for tunnel communicationssystems and the like. Desirable aperture sizes and shapes are described.

12 Claims, 9 Drawing Figures P'A'TE'N'TEDsEP 12 I972 3.691.488

sum 1 or 2 INVENTOR. ALFRED G. HOLTUM, JR.

PATENTED SE! 1 2 we saw 2 or 2 I .NVENTOR ALFRED 6. HOLTUM, JR. BY

Arm-

RADIATING COA CABLE WTHOD F MANUFACT THEOF This invention relates tohigh-frequency transmission elements of the type used for communicationsin tunnels and like purposes.

Various forms of long-length radiating high-frequency transmissionelements have heretofore been proposed, and are in some cases now used,in the fixedstation portion of mobile radio communication systemsdesigned to operate in tunnels, mines, and similar enclosures whereinordinary fixed-station antenna radiators are impractical. Suchlong-length elements serve the combined functions of transmission linesand antennas. The simplest form of such an element is an ordinaryopen-wire transmission line, which inherently has substantial radiationloss. Coaxial lines and waveguides have also been employed, withconventional constructions modified by provision of special forms ofapertures. Such apertured constructions, however, have heretofore beenrelatively complex and expensive to manufacture in order to obtain fullysatisfactory electrical and mechanical characteristics. It is theprincipal object of this invention to provide a simple and easilyfabricated long-lengthradiating transmission element of this generaltype.

The construction of the present invention is obtained by a manner offormation of the apertures which constitutes a simple and inexpensiveaddition to any known process of fabricating transmission elements ofthe type having a corrugated outer conductor. In the present invention,after fabrication of the corrugated tube, there is removed from the tubeat least one angular sector of the corrugation crests, while leaving thecorrugation roots intact, thus producing transverse radiating slotsdistributed along the entire length of the tube. The removal is mostsimply accomplished by a longitudinal milling operation, the corrugatedtube being drawn past the milling tool.

As a further feature of the invention, the aperture dimensioning iscorrelated with the corrugation pitch or spacing in a manner whichavoids necessity for precision of machining in order to produceelectrical and mechanical uniformity despite the small variations incable diameter, etc., which are encountered in economical manufacture.If the selected aperture size is too short in the longitudinal directionof the cable, preciseness of relative positioning of the tool and thetube is found highly critical to uniformity of size of the aperturesproduced. On the other hand, where it is sought to leave only the innerpart of the root portion of the corrugation in the removal operation,excessive precision is required to avoid occasional complete severanceof the corrugation root, thus essentially destroying the mechanicalstrength of the tube in any longitudinal region where this may occur. Inaccordance with this aspect of the invention, the crests are removed toa depth such that the longitudinal spacing between the slots orapertures is between one-half and twice the width (the dimensionlongitudinal of the tube). In this manner uniformity of electrical andmechanical characteristics is greatly improved for any given degree ofprecision of relative positioning of the tool and the tube.

These aspects of the invention, along with its further features andadvantages, will be better understood by consideration of the exemplaryembodiments thereof illustrated in the drawing, in which:

FIG. 1 is a fragmentary top plan view of a radiating high-frequencytransmission element according to the broader aspects of the invention,also showing in more or less schematic form the method of itsfabrication;

FIG. 2 is a transverse sectional view taken along the line 2--2 of FIG.1;

FIG. 3 is a transverse sectional view taken along the line 3-3 of FIG.1;

FIG. 4 is a fragmentary top plan view of another embodiment of theinvention;

FIG. 5 is a transverse sectional view taken along the line 55 of FIG. 4;

FIG. 6 is a transverse sectional view taken along the line 6-6 of FIG.4;

FIG. 7 is a plan view, partially in elevation and partially broken awayin section, of a further embodiment of the invention;

FIG. 8 is 'a fragmentary view illustrating a step in the fabrication ofthe construction of FIG. 4 or 7; and

FIG. 9 illustrates a modification of the fabrication method shown inFIG. 8.

FIGS. 1 through 3 illustrate the invention in its broader basic aspects.A coaxial cable generally indicated at 10 has an inner conductor 12, afoam dielectric 14 and a corrugated outer conductor 16 havingcorrugation crests 18 and corrugation roots 20. The cable may befabricated in any manner, but is preferably of the type produced by acontinuous process of manufacture wherein a strip is continuously formedto the shape of a tube enclosing a foam-dielectric core. In the crests18 are rectangular apertures 22. These are formed by a cutting operationin which the cable is moved past a milling cutter 24 which makes a cutof depth less than the root diameter of the cable, thus leaving theroots intact.

The improved embodiment of FIGS. 4 through 6 is generally similar buthas a number of significant differences. Except for the apertures orslots and the manner of their formation, the cable is of the sameconstruction as that of FIGS. 1 through 3. The apertures of slots 26 arehere of substantially different shape than the apertures or slots 22,being oval-shaped. The manner of formation of the oval-shaped slots isshown in FIG. 8, where it is seen that the apertures 26 are formed by awholly planar cutting or removal operation, similar to the millingoperation of FIGS. 1 through 3 but differing in that a planar cut madeby the straight edge of cutter 27 extends entirely across a chord of thecircular crest. The embodiment of FIGS. 4 through 6 has appreciableadvantages over the rectangular-aperture embodiment of FIGS. 1 through3. The transverse concavity of the rectangular aperture (best seen inFIG. 3) is eliminated, the sole deformation of the external circularshape being a flattened region (FIG. 6). When the illustrated aperturesare enclosed within a protective plastic sheath (to be described inconnection with FIG. 7), the construction of FIGS. 1 to 3 has sharpcutting edges which tend to tear through the sheath when the cableelement is subjected to bending or abrasion. With the improvedconstruction of FIGS. 4 through 6, however, the foam dielectric issubstantially flush with the edges of the apertures and the sheath isbacked by the foam in the region of the apertures, so that the installedsheath is radially supported everywhere.

A further advantage of the construction of FIGS. 4 through 6 lies in therelatively small loss of desirable mechanical properties for any givendegree of radiation leakage permitted by the oval apertures 26 ascompared with the rectangular apertures 22. The theory of radiationthrough such apertures, which are very small compared to a wavelength,is not yet thoroughly known. However, it is known that the amount ofradiation which occurs is not merely a function of the area of theaperture, but varies with both the shape of the aperture and itsorientation with respect to the polarization direction of the energy bywhich it is excited. The leakage efficiency from the oval apertures soformed may be shown to be substantially greater than from rectangularapertures of the same area formed by cutting tools to the same depth.Although the introduction of apertures of any shape necessarilyintroduces some degree of impairment of mechanical characteristics of acorrugated tube, for any specified amount of leakage radiation the lossof desirable mechanical characteristics of the corrugated tubing issubstantially less with the oval-shaped apertures of FIG. 4 than withthe rectangular apertures of FIG. 1.

A variant of the aperturing method of FIG. 8 is shown in FIG. 9. Asthere shown, the cutting-away of the corrugation crests 18 is performedwith a concave milling cutter 30 in place of the linear-edged cutter 27of FIG. 8. The transverse crest shape thus produced in the cutawayregion is intermediate between the original circular arc and the planarflattening of FIG. 8. Such curvature produces, for any given centraldepth of cut, and thus for any given maximum width (in the direction ofcable length) of slot, an oval-shaped slot of greater circumferential orangular extension. It is possible, of

course, to increase the curvature of the cutting edge to the point whereits radius of curvature reaches a semicircular arc matching the crestradius of the cable, but such concavity of the cutting tool isundesirable, as well as being unnecessary.

If so desired, there may be employed a plurality of sets of alignedapertures, although such an embodiment is not illustrated. Likewise, asingle line of apertures may be formed with multiple cutting operationsto extend the arc subtended, as by overlapping of cuts made in themanner of FIG. 8. The use of a curved cutting edge, or of such multiplecuts, is particularly desirable where high leakage radiation fromsmall-diameter cable is wanted.

The embodiments thus far described have been shown as employing a cablehaving annular corrugations, for simplicity of illustration andunderstanding of the principles of the invention, although helicalcorrugations are in more common use. There is shown in FIG. 7 anembodiment manufactured by adding a cutting operation such as shown inFIG. 8 to a commercial cable manufacturing process wherein the cable ishelically, rather than annularly, corrugated. In this embodiment, thehelically corrugated outer conductor 32 has oval-shaped apertures 34similar to those previously described. The shape symmetry of the oval ofFIG. 4 is not lost by the longitudinal component of helical corrugationpitch of FIG. 7, the primary effect of the latter being very slightelongation of the oval with helix angles commonly used. The structure ofFIG. 7 is encased in a plastic sheath 36, as previously mentioned.

For efficiency of radiation, the sheath is constructed of a low-lossdielectric such as polyethylene, preferably omitting all high-lossadditives such as those employed in the black polyethylene commonlyemployed in cable sheathing. Either relatively pure polyethylene orpolyethylene with only low-loss additives (such as the brownpolyethylene used for numerous outdoor applications) may be employed,having no substantial effect on the radiation in thicknesses fullysufficient for protective purposes.

Specific dimensional parameters of both the basic cable construction andthe apertures may of course vary substantially, dependent upon theparticular requirements of use. For use in typical two-way communicationinstallations such as tunnels, subways, and mines, a corrugated cable offrom approximately onehalf inch to approximately 1 inch diameter isfound suitable. In commercial coaxial cable, the corrugations aregenerally sinusoidal in form, as illustrated in the drawing. By formingthe slots or apertures by cutting to a depth such that the bottom of thecut is in the regions of maximum radial slope of the corrugations,variations of slot dimensions due to variations of cutting depth aremade small, and the normal variations of commercial cable diameter alongthe length are readily made to produce only small non-uniformities ofaperture size.

Highly adequate signal strengths can be obtained along such leaky cablewith only so small a portion of the crests cut away that the overallstructure, after installation of the sheath, has substantially the samedesirable mechanical characteristics as ordinary coaxial cable. Thefield intensity radiated varies somewhat with the frequency employed,with any given aperture size. Half-inch and seven-eighths inch radiatingcorrugating cables constructed in accordance with FIG. 7 have beentested at the commonly-used frequencies of and 450 MHz over asubstantial range of aperture sizes produced with a linear-edge millingcutter. With seven-eighths inch cables, cutting depths were usedproducing oval apertures of from 0.132 to 0.317 inch major axis. Theseleakage-aperture sizes correspond to a wide range of performancerequirements for two-way communication systems with mobile transmittingand receiving units of varying receiver sensitivities and transmitterpower. For communication with mobile units of high receiver sensitivityand high transmitter power, half-inch cables with oval apertures of0.175 inch major axis can sufi'ice, particularly in the higher frequencyband. However, where substantially higher radiation is required,apertures of adequate size cannot in general be produced with a singleplanar cut in halfinch cable; in such case, fully adequate signalstrength is obtained by intersecting planar cuts or by the employment ofa concave milling cutter, to produce apertures of major axiscorresponding to a chord-length of somewhat greater than 0.2 inch.

As is conventional in the art, the properties of the transmissionelement have herein been described in terms of radiation, leakage, etc.As in the case of prior devices for the same purpose, it will beunderstood that such terms describe the properties when the element isemployed for reception of ambient signals as well as for radiation ofsignals fed to the cable in conventional fashion.

Persons skilled in the art will readily devise numerous embodiments ofthe method and product of the invention differing substantially from theembodiments herein described, but nevertheless utilizing the invention,particularly in its broader aspects. As one example, the broaderteachings of the invention may be applied to corrugated waveguideexcited in any mode wherein the direction of wall-current flow islongitudinal, i.e., similar to the coaxial cable transmission mode inthis respect. As another example, gradation of aperture size may beproduced along the length by mere adjustment of the cutting depth as thecable is moved past the cutting station; such gradation may be employedwhere the radiating cable is to be used in such long length, and withsuch requirement of uniformity of radiation, as to make it desirable tocompensate for attenuation along the length.

The invention should accordingly not be considered limited by theparticular embodiments illustrated and described, and the protectionafiorded thereto should extend to all utilization of the invention asdefined in the claims below, and equivalents thereto.

What is claimed is:

l. A radiating high-frequency transmission element for tunnelcommunications and the like comprising an elongated transverselycorrugated conducting tube having longitudinally aligned apertures inall corrugation crests, the corrugation roots being unapertured.

2. The radiating transmission element of claim 1 wherein the conductingtube is the outer conductor of a coaxial cable.

3. The radiating transmission element of claim 2 having a foamdielectric within the outer conductor.

4. The radiating transmission element of claim 1 having a low-lossdielectric sheath encasing the corrugated tube.

5. The radiating transmission element of claim I having the apertures ofgenerally oval shape.

6. The radiating transmission element of claim 1 comprising a corrugatedgenerally circular tube having at least one portion of the circularperiphery thereof cut away from circularity along its entire length toform the apertures.

7. The transmission element of claim 6 wherein the transverse shape isfree of concavities.

8. The transmission element of claim 6 wherein the apertures are of awidth from one-half to twice the distance of the spacing therebetween.

9. The radiating high-frequency transmission element of claim 1comprising a corrugated foam-dielectric coaxial cable having generallyoval-shape apertures in the corrugation crests and enclosed in alow-loss dielectric sheath.

10. In a method of making a radiating high-frequency transmissionelement for tunnel communications and the like, the improvementcomprising removing at least one angular sector of all the corrugationcrests of a corrugated high-frequency transmission element while leavingthe corrugation roots intact.

11. The improved method of claim 10 wherein the transmission element issubstantially circular and said sector of the corrugation crests isremoved by cutting away a circumferential portion leaving the transverseshage free of concavity.

. The improved method of claim 10 including the added step of thereafterencasing the element in a lowloss dielectric sheath.

1. A radiating high-frequency transmission element for tUnnelcommunications and the like comprising an elongated transverselycorrugated conducting tube having longitudinally aligned apertures inall corrugation crests, the corrugation roots being unapertured.
 2. Theradiating transmission element of claim 1 wherein the conducting tube isthe outer conductor of a coaxial cable.
 3. The radiating transmissionelement of claim 2 having a foam dielectric within the outer conductor.4. The radiating transmission element of claim 1 having a low-lossdielectric sheath encasing the corrugated tube.
 5. The radiatingtransmission element of claim 1 having the apertures of generally ovalshape.
 6. The radiating transmission element of claim 1 comprising acorrugated generally circular tube having at least one portion of thecircular periphery thereof cut away from circularity along its entirelength to form the apertures.
 7. The transmission element of claim 6wherein the transverse shape is free of concavities.
 8. The transmissionelement of claim 6 wherein the apertures are of a width from one-half totwice the distance of the spacing therebetween.
 9. The radiatinghigh-frequency transmission element of claim 1 comprising a corrugatedfoam-dielectric coaxial cable having generally oval-shape apertures inthe corrugation crests and enclosed in a low-loss dielectric sheath. 10.In a method of making a radiating high-frequency transmission elementfor tunnel communications and the like, the improvement comprisingremoving at least one angular sector of all the corrugation crests of acorrugated high-frequency transmission element while leaving thecorrugation roots intact.
 11. The improved method of claim 10 whereinthe transmission element is substantially circular and said sector ofthe corrugation crests is removed by cutting away a circumferentialportion leaving the transverse shape free of concavity.
 12. The improvedmethod of claim 10 including the added step of thereafter encasing theelement in a low-loss dielectric sheath.